This invention pertains to photovoltaic devices, particularly, to solar cells comprising amorphous silicon. More particularly, this invention relates to photovoltaic devices comprising amorphous silicon wherein the amorphous silicon in the form of an intrinsic layer or i-layer is thick, and wherein the photovoltaic devices having such thick i-layers perform unexpectedly well at elevated temperatures.
Solar cells and other photovoltaic devices convert solar radiation and other light into usable electrical energy. The energy conversion occurs as the result of the photovoltaic effect. Solar radiation (sunlight) impinging on a photovoltaic device and absorbed by an active region of semi-conductor material, e.g. an intrinsic i-layer of amorphous silicon, generates electron-hole pairs in the active region. The electrons and holes are separated by an electric field of a junction in the photovoltaic device. The separation of the electrons and holes by the junction results in the generation of an electric current and voltage. The electrons flow toward the region of the semiconductor material having an n-type conductivity. The holes flow toward the region of the semiconductor material having a p-type conductivity. Current will flow through an external circuit connecting the n-type region to the p-type region as long as light continues to generate electron-hole pairs in the photovoltaic device.
Single-junction photovoltaic devices comprise three layers. These are p- and n-layers which are extrinsic or doped and the i-layer which is intrinsic or undoped (at least containing no intentional doping). The i-layer is thicker than the doped layers. This is because mainly light absorbed in the i-layer is converted to electrical power which can be used in an external circuit. As discussed above, when a photon of light is absorbed in the i-layer it gives rise to a unit of electrical current (an electron-hole pair). However, this electrical current will go nowhere on its own. Hence, the p- and n-layers. These layers, which contain charged dopant ions, set up a strong electric field across the i-layer. It is this electric field, which xe2x80x9cdrawsxe2x80x9d the electric charge out of the i-layer and sends it through an external circuit where it can provide power for electrical components.
Thin film solar cells, which are one type of solar cells, are typically constructed of amorphous silicon-containing semiconductor films on a substrate. The substrate of the solar cell can be made of glass or a metal, such as aluminum, niobium, titanium, chromium, iron, bismuth, antimony or steel. Soda-lime glass is often used as a substrate because it is inexpensive, durable and transparent. If a glass substrate is used, a transparent conductive coating, such as tin oxide (SnO2) can be applied to the glass substrate prior to forming the amorphous silicon-containing semiconductor films. A metallic contact can be formed on the back of the solar cell. Solar cells are often placed in metal frames to provide attractive photovoltaic modules.
In an amorphous silicon-containing solar cell, the amorphous silicon component is comprised of a body of hydrogenated amorphous silicon (a-Si:H) material. This can be formed in a glow discharge of silane (SiH4). Such cells can be of the type described in U.S. Pat. No. 4,064,521 entitled Semiconductor Device Having A Body Of Amorphous Silicon, which issued to David E. Carlson on Dec. 20, 1977.
The process of glow discharge of silane and other suitable materials involves the discharge of energy through a gas at relatively low pressure and high temperature in a partially evacuated chamber. A typical process for fabricating an amorphous silicon solar cell comprises placing a substrate on a heated element within a vacuum chamber. A screen electrode, or grid, is connected to one terminal of a power supply, and a second electrode is connected to the substrate. While silane, at low pressure, is admitted into the vacuum chamber, a glow discharge is established between the two electrodes and an amorphous silicon film deposits upon the substrate.
Amorphous hydrogenated silicon (a Si:H) based solar cell technology is a good candidate for large area, low-cost photovoltaic applications. The basic device structure is a single p-i-n junction or an n-i-p junction in which all layers are traditionally amorphous and are made in a continuous plasma deposition process as described above. The collection of layers resulting in a p-i-n or n-i-p component, are referred to herein, at times, as a cell.
Current output of a photovoltaic device is maximized by increasing the total number of photons of differing energy and wavelength, which are absorbed by the semiconductor material. The solar spectrum roughly spans the region of wavelength from about 300 nanometers to about 2200 nanometers, which corresponds to from about 4.2 eV to about 0.59 eV, respectively. The portion of the solar spectrum, which is absorbed by the photovoltaic device is determined by the size of the bandgap energy of the semiconductor material. Solar radiation (sunlight) having an energy less than the bandgap energy is not absorbed by the semiconductor material and, therefore, does not contribute to the generation of electricity, current, voltage and power, of the photovoltaic device.
The doped layers in the device play a key role in building up the strong internal electric field across the i-layer, which is the predominant force in collecting photocarriers generated in the i-layer. An important quality for the doped layers used in solar cells, besides good electrical properties, is low optical absorption. In contrast to single crystalline devices where p-n junctions can be used, photons absorbed in amorphous doped layers can be lost because the diffusion length of photo-carriers is extremely short in those layers. This requirement is especially important for the p-layer through which light enters into the device. It is partly for this reason that amorphous silicon carbon (a-SiC:H) p-layers with an optical bandgap of about 2.0 eV have been used instead of amorphous silicon.
On of the most important objectives in designing and manufacturing a photovoltaic device such as a solar cell is to maximize the efficiency of the device in converting light energy into electric energy. It would be desirable in some applications to use the photovoltaic device in a climate, or under other conditions, as on a roof top with poor or no air circulation around the device. Under such conditions, the photovoltaic device may reach elevated temperatures of, for example, more than 50xc2x0 C., or more than 60xc2x0 C. It would be advantageous to have a photovoltaic device that operates efficiently at these elevated temperatures and, more particularly, it would be highly advantageous to have an amorphous silicon-type photovoltaic device, which operates more efficiently at these high temperatures compared to prior devices. The present invention provides for such photovoltaic devices and a process for using them to generate electricity.
This invention is a photovoltaic device comprising an amorphous silicon-containing i-layer that is efficient at elevated operation temperatures. This invention is a photovoltaic device comprising an amorphous, silicon-containing i-layer wherein the i-layer has a thickness of at least about 3000 xc3x85, more preferably at least about 3500 xc3x85. Preferably the i-layer has a thickness of about 3000 to about 5500 xc3x85, more preferably about 3500 to about 5000 xc3x85. It can have a thickness of about 4000 to about 5000 xc3x85, or from greater than 4000 xc3x85 and, suitably, up to about 5000 or 6000 xc3x85. The i-layer can be at least about 5000 xc3x85, at least about 5500 xc3x85, and can be at least about 6000 xc3x85. We have determined that photovoltaic devices having such thick i-layers function much more efficiently at elevated operation temperatures, for example, temperatures of at least about 50xc2x0 C., than prior photovoltaic devices having thinner amorphous i-layers. The photovoltaic devices of this invention preferably have a p-layer of less than 150 xc3x85, more preferably no more than about 100 xc3x85 and most preferably no more than about 80 xc3x85 in thickness. P-layers of about 60 to about 80 xc3x85, or about 70 to about 80 xc3x85 in thickness are also highly preferred. We have determined that these relatively thin p-layers are more preferable for operation at high temperatures, especially at the higher temperatures mentioned above. This invention is also a process for generating electric current comprising exposing photovoltaic devices to light, particularly solar radiation, wherein the photovoltaic device is operated at elevated temperatures, for example temperatures of at least about 50xc2x0 C., more preferably temperatures of at least about 55xc2x0 C., and most preferably at temperatures of at least about 60xc2x0 C. wherein the photovoltaic device comprises an amorphous, silicon-containing i-layer having a thickness of at least about 3000 xc3x85, more preferably at least about 3500 xc3x85. The upper temperature limit can be 90xc2x0 C. or even 100xc2x0 C., or higher. Preferably the i-layer has a thickness of about 3000 to about 5500 xc3x85, more preferably about 3500 to about 5000 xc3x85. In the process of this invention the p-layers the photovoltaic devices preferably have a p-layer of less than 150 xc3x85, more preferably no more than about 100 xc3x85 and most preferably no more than about 80 xc3x85 in thickness. P-layers of about 60 to about 80 xc3x85 in thickness, or about 70 to about 80 xc3x85 in thickness are also highly preferred in the photoelectric devices in the process of this invention.
In the preferred photovoltaic device of this invention, a single or dual layer front contact is positioned on a substrate and at least one amorphous silicon-containing thin film semiconductor is deposited on the front contact. The substrate can be stainless steel or other metal, but preferably comprises a glass substrate. Advantageously, a dual layer back (rear) contact is deposited on the thin film semiconductor. One of the layers of each of the contacts can comprise a wide band gap semiconductor, preferably a transparent metallic conductive oxide, such as tin oxide, indium-tin oxide, zinc oxide, or cadmium stannate. Desirably, the other layer of the dual layer back contact comprises a metal, such as: aluminum, silver, molybdenum, platinum, steel, iron, niobium, titanium, chromium, bismuth, antimony, or oxides of the preceding. The front contact can also have a second layer comprising a dielectric on a glass substrate, such as silicon dioxide.
The thin film semiconductor preferably comprises an amorphous silicon-containing material, such as: hydrogenated amorphous silicon, hydrogenated amorphous silicon carbon, or hydrogenated amorphous silicon germanium. Most preferably it is amorphous silicon. The photovoltaic modules preferably comprise monolithic solar cells, such as: single junction solar cells, multi-junction solar cells, dual junction (tandem) solar cells or triple junction solar cells. If desired, a portion of the photovoltaic module can be microcrystalline or polycrystalline. Most preferably, the thin film semiconductor comprises a single junction solar cell.
A conductive paste, such as a fritted conductive paste, can be deposited on the substrate and then heated and bonded to the substrate to provide a means of collecting current and to allow a solderable external connection.
In the preferred process for making the photovoltaic devices of this invention, the transparent conductive oxide of the front contact is laser scribed in a pattern of parallel-spaced scribes, and the semiconductor layers are deposited on the front contact by enhanced plasma chemical vapor deposition. Thereafter, the transparent conductive oxide first layer of the dual layer rear contact is deposited on the rear (back) layer of the semiconductor. Advantageously, the semiconductor layers and the transparent conductive oxide (but not the conductive oxide of the front contact) first layer of the dual layer rear contact are simultaneously laser scribed in a pattern parallel to the front pattern on the front contact and a trench is formed. Subsequently, a metallic second layer on the rear contact is deposited on the transparent conductive oxide first layer of the rear contact. Desirably, the metal of the second layer fills the trench and provides a superb mechanical and electrical interconnect (interconnection) between the front contact and the metallic second layer of the rear contact. The metallic second layer of the rear contact is then laser scribed in a pattern parallel to the pattern on the semiconductor and first layer of the rear contact to isolate the segments of the photovoltaic module and provide series interconnects for the dual layers of the rear contact. A perimeter can then be laser scribed around the photovoltaic module to completely isolate the segments of the photovoltaic module from each other, except through the interconnects. The photovoltaic module can then be cured by generating a direct current across adjacent segments in a reverse bias orientation to remove shunts and shorts and decrease parasitic losses.
An encapsulating material, such as ethyl vinyl acetate (EVA) and/or Tedlar type plastic can be deposited on the back second layer of the rear contact, and a superstrate can be laminated to the photovoltaic module to provide enhanced environmental protection for the photovoltaic module.
In the preferred form, the superstrate, if any, and substrate are glass, the transparent conductive oxide of the front contact is tin oxide, the transparent conductive oxide first layer of the rear contact is zinc oxide and the metallic second layer of the rear contact and the interconnect is aluminum. The semiconductor preferably comprises a single junction cell comprising an amorphous silicon p-i-n or n-i-p cell. However, this invention is not so limited and the semiconductor can comprise other semiconductors such as a dual junction cell wherein the semiconductor comprises, for example, an amorphous silicon p-i-n or n-i-p cell and a silicon germanium p-i-n or n-i-p cell.
The inventive monolithic solar cells are able to capture a broader spectrum of sun light and convert and harness a greater amount of solar energy into electricity at elevated temperatures compared to prior cells. Advantageously, the monolithic solar cells and process for their production are efficient, effective, reliable, and economical. As used in this application, the term xe2x80x9cmonolithicxe2x80x9d means a solar cell comprising a front contact and a rear-contact. The segments, layers, or cells of a monolithic solar cell are electrically and optically connected to each other to form one solar cell or photovoltaic module.
The transparent conductive oxide, such as zinc oxide or tin oxide, can be deposited by low-pressure chemical vapor deposition (LP CVD). The layer of doped and undoped amorphous silicon and microcrystalline amorphous silicon can be deposited by enhanced plasma chemical vapor deposition (EP CVD), also referred to as plasma enhanced chemical vapor deposition (PE CVD). For dual junction (tandem) p-i-n/p-i-n solar cells, an amorphous silicon germanium semiconductor can be deposited by PECVD to form the i-layer of the bottom p-i-n junction, and an amorphous silicon conductor can be deposited by PECVD to form the i-layer of the front junction. A tunnel junction or recombination junction connects the back amorphous silicon germanium p-i-n cell to the front amorphous silicon p-i-n cell.
The multi-junction solar cells can also be fabricated by forming a microcrystalline sandwich with a n-type semiconductor (conductor) comprising a microcrystalline tunnel junction layer between a polycrystalline solar cell and an adjoining n-i-p amorphous silicon-containing solar cell. The amorphous silicon-containing solar cell has a positively doped p-layer, an active instrinic i-layer, and a negatively doped n-layer. A tunnel junction or recombination junction connects the polycrystalline back solar cell to the amorphous silicon-containing solar cell. The tunnel junction can comprise a negatively doped layer from one of the solar cells, a positively doped layer from the other solar cell, and at least one intermediate tunnel junction layer positioned between the polycrystalline and amorphous silicon-containing second solar cells.
The tunnel junction layer can be fabricated by etching and treating one of the doped layers in the tunnel junction to form an etched surface thereon and, thereafter, nucleating from the etched surface to form and grow a microcrystalline tunnel junction layer. As used in this application, the term xe2x80x9cnucleatingxe2x80x9d means the initial growth phase of microcrystalline layers. Etching can comprise enhanced plasma chemical vapor deposition with an etchant (treatment material), such as hydrogen, deuterium, HD, helium, and argon. Preferably, etching comprises hydrogen etching alone by DC or RF enhanced plasma chemical vapor deposition while preventing substantial optical and electrical damage to the doped layers. Desirably, for best results, silane or other feedstock is prevented from being deposited with the etchant.
Nucleation from the etched surface can be provided to accelerate microcrystalline growth. Microcrystalline nucleation can be accomplished by enhanced plasma chemical vapor deposition with a dopant and a feedstock diluted with a diluent. The dopant can be: a negative dopant comprising a n-type dopant, such as phosphine (PH3) or other phosphorous-containing compounds; or a positive dopant comprising a p-type dopant, such as diborane (B2H6), BF3, or other boron-containing compounds. The feedstock can be: silane (SiH4), disilane (Si2H6), tetramethyl silane, Si(CH3)4, SiF4, SiHF3, SiH2Cl2, CHN(SiH3)4xe2x88x92N wherein N is an integer in the range of 0 to 3, a carbon based feedstock, or a germanium based feedstock. The feedstock can also have the general formula SiNH2N+2xe2x88x92M YM wherein:
Si=silicon
H=hydrogen or deuterium
Y=a halogen [fluorine (F), chlorine (Cl), bromine (Br), Iodine (I), etc.]
N=positive integerxe2x89xa71
M=positive integer; and
2N+2xe2x88x92Mxe2x89xa70.
The diluent can be hydrogen (H2), deuterium (D2), or HD. The dilution ratio of the diluent to the feedstock can range from about 50:1 to about 200:1.
Plasma enhanced chemical vapor deposition (PECVD) can be by: cathodic direct current (DC) glow discharge, anodic DC glow discharge, radio frequency (RF) glow discharge, very high frequency (VHF) glow discharge, alternating current (AC) glow discharge, or microwave glow discharge. Plasma enhanced chemical vapor deposition of microcrystalline layers can be accomplished at a temperature ranging from 80-300xc2x0 C., at a pressure ranging from 0.5-5 Torr, with a dilution ratio of diluent to the feedstock (deposition gas) ranging from 20:1 to 200:1.
The tunnel junction of the multi-junction solar cell can have an etched surface and at least one microcrystalline tunnel junction layer sandwiched between the doped layer of one solar cell and an opposite doped layer of the other solar cell. The etched surface can be a hydrogen plasma etched surface, such as a n-doped amorphous silicon surface or a p-doped amorphous silicon surface. The microcrystalline tunnel junction layer can be a p-type microcrystalline layer and/or an n-type microcrystalline layer and the microcrystalline layer can be microcrystalline silicon carbon, microcrystalline silicon germanium, or microcrystalline silicon. The microcrystalline layer can have a thickness of 50-120 xc3x85, preferably from 80-100 xc3x85. The tunnel junction can comprise an n-type doped non-crystalline amorphous layer, an n-type microcrystalline tunnel junction layer, and a p-type doped non-crystalline amorphous layer. Tunnel junction can also comprise a p-type microcrystalline tunnel junction layer in lieu of or in addition to the n-type microcrystalline tunnel junction layer.
A more detailed explanation of the invention is provided in the following description and appended claims oaken in conjunction with the accompanying drawings.